During flight operations, engine fuel is used to control or govern engine power and propeller pitch is used to control or govern propeller speed. In systems currently in use, it is common to utilize a propeller underspeed governor to increase and a propeller overspeed governor to decrease engine power so that engine delivered power is matched to the propeller absorbed power.
FIG. 1 shows a typical prior art overspeed governor and underspeed governor. As can be seen from FIG. 1, propeller speed NP is sensed, compensated and then subtracted from a propeller overspeed reference (NPOSRF). The summing junction output is multiplied by a gain (KOSG) then sent to the fuel control logic. FIG. 1 also shows the same control scheme but configured for the underspeed governor.
In control systems such as those depicted in FIG. 1, a commanded increase in power lever angle (PLA) requires that the engine power be scheduled to a higher power. Increasing engine power however will cause propeller speed to increase, requiring the propeller speed governor to increase pitch in response to this overspeed. The problem with this prior art overspeed governor is that the propeller governor speed error needed for maximum increase pitch rate is at a propeller speed near to the propeller speed reference in the fuel control overspeed governor, so that the fuel control overspeed governor prevents a rapid increase in engine gas generator speed and power. Therefore, the combination of propeller pitch governing propeller speed and engine fuel providing propeller overspeed governing will often result in sluggish engine gas generator acceleration. Similarly, the underspeed governor depicted in FIG. 1, which is the combination of propeller pitch governing propeller speed and engine fuel providing propeller underspeed governing, often will result in sluggish engine gas generator deceleration.